In Journal of the Electrochemical Society: Extended Abstracts. Vol. 86-1, pages 337-338, Abstract no. 232, entitled "A Newly Developed Silicon to Silicon Direct Adhesion Method"proposes a method of making an adhesion-based connection between two silicon wafers. For this purpose, the wafers are first polished, and their surfaces are "washed" or chemically treated for hydrophilization. Subsequently the prepared surfaces of the wafers to be connected are brought together at room temperature. The bonding process, named Silicon Fusion Bonding (SFB) is ended with temperature treatment of the wafers at about 1000.degree. C. In this process, it is unnecessary to subject the wafers to high pressure. It is also unnecessary to apply voltage between the two wafers.
For chemical treatment of wafers for the bonding process, chemical solutions are specified, which would destroy a sensor system with completed, integrated circuitry, especially one with aluminum conductive tracks. Such treatments disclosed in the literature include:
H.sub.2 SO.sub.4 -H.sub.2 O.sub.2 (sulfuric acid//hydrogen peroxide) and HC1-H.sub.2 O.sub.2 ; (hydrochloric acid/hydrogen peroxide) PA0 aqua regia; PA0 RCA cleaning (RCA cleaning is a standard cleaning process for silicon wafers. Two cleaning solutions are used in succession. The first consists of H.sub.2 O, H.sub.2 O.sub.2 (30%), and NH.sub.4 OH in the ratio 5:1:1. The second consists of H.sub.2 O, H.sub.2 O.sub.2 (30%), and HCI (37%) in the ratio 6:1:1; PA0 KOH (potassium hydroxide); PA0 Choline; PA0 hot (about 60.degree.-80.degree. C.) or boiling (about 120.degree. C.) nitric acid [HNO.sub.3 (65%)]; PA0 Solution of H.sub.2 O.sub.2 and NH.sub.4 OH at 50.degree.-60.degree. C. PA0 polishing and cleaning surfaces of silicon wafers, PA0 dipping the wafers in fuming nitric acid, e.g., 100% HNO.sub.3, then rinsing the wafers with de-ionized water and drying the wafers; PA0 assembling the thus-polished surfaces of the wafers together in a substantially particle-free atmosphere, and PA0 subjecting the assembled wafers to a temperature treatment to form a secure silicon-to-silicon bond, PA0 the temperature treatment being carried out at a temperature between about 100.degree. C. and about 400.degree. C. PA0 a first silicon wafer bearing electrical circuit elements and at least one conductive track, and PA0 a second silicon wafer secured to the first wafer, at least one of the wafers being formed with a micromechanical structural element produced by the steps of PA0 cleaning and polishing respective surfaces of the first and second wafers, PA0 rendering the polished surfaces hydrophilic by dipping the wafers in fuming nitric acid, e.g., 100% HNO.sub.3, PA0 assembling the hydrophilic surfaces together, and PA0 temperature-treating the assembled wafers at a temperature between 100.degree. C. and 400.degree. C.